1. Field of the Invention
The present invention relates to an inductive element such as a thin-film inductor or a thin-film transformer. More particularly, the invention relates to an inductive element which has a magnetic layer formed of a plurality of magnetic films in lamination, can avoid concentration of magnetic fluxes from a coil layer to the magnetic film the closest to the coil layer, or more preferably, can improve high frequency characteristic by ensuring formation of a uniform magnetic flux distribution for all the magnetic layers.
2. Description of the Related Art
For example, a thin-film inductor comprises a first magnetic layer formed on a substrate, a coil layer formed via an insulating layer on the first magnetic layer, and a second magnetic layer formed via another insulating layer on the coil layer.
The above-mentioned thin-film inductor, which is used, for example, in a micro DC-DC converter, has a problem in that, when the magnetic layers composing the thin-film inductor are formed by single layers having a large thickness, eddy-current loss increases in a high frequency band, thus making it impossible to obtain a satisfactory high frequency characteristic.
In a conventional art, therefore, a thin-film inductor 1 comprising magnetic layers 3 and 5 formed into a multi-layer structure was devised as shown in FIG. 10. This permits maintenance of inductance at above a certain level and reduction of eddy-current loss, thus improving high frequency characteristic.
As shown in FIG. 10, a coil layer 4 is provided between a first magnetic layer 5 and a second magnetic layer 3, and each of the magnetic layer 5 and 3 comprises a plurality of magnetic films 5a to 5c or 3a to 3c in lamination.
Also as shown in FIG. 10, all the magnetic films composing the magnetic layers 5 and 3 are formed with the same width t1, the same thickness h1 and the same magnetic permeability xcexc1.
However, if all the magnetic films forming the magnetic layers 5 and 3 are formed with the same width t1, the same thickness h1 and the same magnetic permeability t1, a magnetic film closer to the coil layer 4 would have concentration of magnetic fluxes from the coil layer 4. This prevents formation of a uniform magnetic flux density distribution throughout the entire magnetic films, thus making it impossible to cause all the magnetic films to perform the same functions.
Because all the magnetic films forming the magnetic layer 4 are formed into the same thickness h1, the magnetic films 5a and 3a the closest to the coil layer 4 have an increased magnetic flux density within the magnetic films caused by fluxes generated from the coil layer 4, thus making it easier to reach the magnetic saturation state. When the magnetic films 5a and 3a become magnetization-saturated, the magnetic films 5a and 3a no more substantially function as magnetic films but are in a state similar to air-core state.
More specifically, it was the conventional practice to reduce eddy-current loss by forming the magnetic layer through lamination of thin magnetic films. However, if all the magnetic films are formed with the same thickness h1 and the same magnetic permeability xcexc1, magnetic fluxes from the coil layer 4 are concentrated more on a magnetic film closer to the coil layer 4, and the magnetic film approaches the saturated state more along with the increase in amplitude of excitation current or in superposed DC current, thus posing problems such as a decrease in inductance.
Magnetic fluxes from the coil layer 4 concentrate more on a magnetic film closer to the coil layer 4, thus preventing obtaining a uniform magnetic flux distribution for the individual magnet films. This is proved also from equivalent resistance of the individual magnetic films as described below.
FIG. 11 is a graph illustrating the relationship between frequency and equivalent resistance of the individual magnetic films. In all frequency bands, as shown in FIG. 11, equivalent resistance is the largest in the magnetic films 5a and 3a the closest to the coil layer 4, followed by equivalent resistance of the magnetic films 5b and 3b, and equivalent resistance is smaller in the magnetic films 5c and 3c the most distant from the coil layer 4.
The differences in equivalent resistance as described above between the magnetic films are attributable to different amounts of magnetic flux induced from the coil layer 4 to the magnetic films. More magnetic fluxes generated from the coil layer 4 are induced into the magnetic film closer to the coil layer 4. A magnetic film closer to the coil layer 4 would have therefore a larger equivalent resistance.
As described above, when the magnetic films 5a and 3a the closest to the coil layer 4 reach the magnetically saturated state, the function to be performed by the magnetic films 5 and 3 is accomplished by only the magnetic films 5b, 3b, 5c and 3c, and this results in substantially decrease in the thickness of the magnetic films 5 and 3. This in turn leads to a decrease in inductance and a deterioration of superposed DC current characteristic.
When the magnetic films 5a and 3a reach the magnetically saturated state, fluxes generated from the coil layer 4 are concentrated on the magnetic films 5b and 3b which are closer to the coil layer 4 next to the magnetic films 5a and 3a, and the magnetic films 5b and 3b become susceptible to magnetic saturation as in the case of the magnetic films 5a and 3a, and this forms another factor causing a deterioration of superposed DC current characteristic.
The present invention was developed to solve these problems and has an object to provide an inductive element which has a magnetic layer formed of a plurality of magnetic films in lamination, can avoid concentration of magnetic fluxes from a coil layer to the magnetic film the closest to the coil layer, or more preferably, can improve high frequency characteristic by ensuring formation of a uniform magnetic flux distribution for all the magnetic layers.
A first aspect of the invention provides an inductive element comprising a coil layer, and a magnetic layer covering the coil layer via an insulating layer; the magnetic layer being formed of at least two superposed magnetic layers; and inductive element including a combination of magnetic films in which, for any two magnetic films forming the magnetic layer, the value of product of multiplication of magnetic permeability and thickness of a magnetic film closer to the coil layer is smaller than the value of product of multiplication of magnetic permeability and thickness of a magnetic film more distance from the coil layer.
When forming a magnetic layer into a multi-layer configuration, it is the conventional practice to form a plurality of magnetic films with the same magnetic permeability and the same thickness. As a result, fluxes from the coil layer are concentrated more on a magnetic film closer to the coil layer, thus preventing formation of a uniform magnetic flux distribution for all magnetic films, and the individual magnetic films cannot equally display their functions. Particularly, the magnetic film the closest to the coil layer becoming in magnetically saturated state and cannot substantially function as a magnetic layer, leading to decreased superposed DC current characteristic and inductance. The present circumstances are therefore that the effect of using a multi-layer magnetic layer is not sufficiently displayed.
In the first aspect of the invention, in contrast, concentration of magnetic fluxes on a magnetic film close to the coil layer can be avoided by using, for any two magnetic films forming the magnetic layer, a value of product of multiplication of magnetic permeability and thickness of a magnetic film closer to the coil layer smaller than the value of product of multiplication of magnetic permeability and thickness of a magnetic film more distant from the coil layer.
When the magnetic layer has at least three superposed magnetic films, in order to avoid concentration of magnetic fluxes from the coil layer on a magnetic film close to the coil layer and to make fluxes easily inducible to a magnetic film more distant from the coil layer, it is preferable that the value of product of multiplication of magnetic permeability and thickness of the magnetic film the closest to the coil layer is smaller than the value of product of multiplication of magnetic permeability and thickness of the other magnetic films. It is more preferable that the value of product of multiplication of magnetic permeability and thickness of the magnetic film the most distant from the coil layer is larger than the value of product of multiplication of magnetic permeability and thickness of the other magnetic films. Further, it is the most desirable that the value of product of multiplication of magnetic permeability and thickness of the magnetic films is sequentially larger according as the distance from the coil layer becomes longer.
More specifically, in the first aspect of the invention, it is possible to avoid concentration of magnetic fluxes from the coil layer to the magnetic film the closest to the coil layer, increase the amount of fluxes induced to the magnetic film more distant from the coil layer, and induce an amount of fluxes meeting the individual magnetic films from the coil layer. It is hence possible to bring the magnetic flux density distribution of each magnetic film closer to the uniform state, thus ensuring substantially equal functioning of all the magnetic films. Further, by appropriately adjusting the value of product of multiplication of magnetic permeability and thickness of each magnetic film, it is possible to reduce the total sum of values of equivalent resistance of the individual magnetic films, improve superposed DC current characteristic as compared with the conventional art, and prevent inductance from decreasing. It is also possible to reduce iron loss, and particularly, to reduce iron loss in the high frequency region.
The first aspect of the invention utilizes the fact that a difference in thickness between the magnetic films forming the magnetic layer leads to different values of product of multiplication of magnetic permeability and thickness. If the individual magnetic films are identical in magnetic permeability, it is possible to produce a difference in the value of product of multiplication of magnetic permeability and thickness by using different thicknesses for the individual magnetic films. The first aspect of the invention is based also on the fact that a difference in magnetic permeability between the magnetic films forming the magnetic layer leads to different values of product of multiplication of magnetic permeability and thickness. If all the magnetic films have the same thickness, it would be possible to produce differences in the value of product of multiplication of magnetic permeability and thickness by using different values of magnetic permeability for the individual magnetic films. By appropriately adjusting thicknesses and selecting magnetic materials, therefore, it is possible to obtain more uniform magnetic flux distributions for the individual magnetic films, and ensure substantially equal functioning of all the magnetic films.
A second aspect of the invention provides an inductive element comprising a coil layer, and a magnetic layer covering the coil layer via an insulating layer, wherein the magnetic layer is formed of at least two superposed magnetic films, and of any two magnetic films from among those forming the magnetic layer, the magnetic layer includes a combination in which the magnetic film closer to the coil layer has a thickness smaller than the thickness of the magnetic film more distant from the coil layer.
When the magnetic layer has at least three superposed magnetic films, the thickness of the magnetic film the closest to the coil layer should preferably be smaller than the thickness of the other magnetic films, and the magnetic film the most distant from the coil layer should preferably be larger than the thickness of the other magnetic films. More preferably, the thickness of the magnetic films should be gradually larger according as the film becomes more distant from the coil layer.
A third aspect of the invention provides an inductive element comprising a coil layer and a magnetic layer covering the coil layer via an insulating layer; the magnetic layer having at least two superposed magnetic films, including a combination of magnetic films in which magnetic permeability of a magnetic material forming the magnetic film close to the coil layer is smaller than magnetic permeability of a magnetic material forming the magnetic film distant from the coil layer, for any two magnetic films forming the magnetic layer.
When the magnetic layer has at least three superposed magnetic films, magnetic permeability of a magnetic material forming the magnetic film the closest to the coil layer should preferably be smaller than magnetic permeability of a magnetic material forming the other magnetic films, and magnetic permeability of a magnetic material forming the magnetic film the most distant from the coil layer should preferably be larger than magnetic permeability of a magnetic material forming the other magnetic films. More preferably, magnetic permeability of the magnetic material forming the magnetic films should be sequentially larger according as the magnetic film is more distant from the coil layer.
In the aforementioned aspect of the invention, the magnetic film the closest to the coil layer from among the magnetic films forming the magnetic layer should preferably have both side ends located on the center side more than the both side ends of at least one of the other magnetic films, and the magnetic film the most distant from the coil layer from among the magnetic films forming the magnetic layer should preferably have both side ends extending sideways more than both side ends of at least one of the other magnetic films. It is the most desirable that, for any two magnetic films from among the magnetic films forming the magnetic layer, a magnetic film more distant from the coil layer has a larger width so that the both side ends of a magnetic film more distant from the coil layer extend sideways more than the both side ends of a magnetic film closer to the coil layer.
As a result, it is possible to more appropriately avoid concentration of magnetic fluxes from the coil layer to the magnetic film the closest to the coil layer, and further, makes it easier for magnetic fluxes from the coil layer more inducible according as the magnetic film becomes more distant from the coil layer, thus permitting achievement of a more uniform magnetic flux distribution for the individual magnetic films and makes it possible to cause all the magnetic films to appropriately function.
In the aforementioned aspect of the invention, from among the magnetic films forming the magnetic layer, a slit or a notched portion should preferably be formed in at least one magnetic film except for the magnetic film the most distant from the coil layer. The most distant magnetic film plays the role of a shield, and by forming a slit or a notched portion in at least one magnetic film except for the magnetic film the most distant from the coil layer, it is possible to prevent magnetic fluxes from concentrating on the magnetic layer the closest to the coil layer, achieve a uniform magnetic flux density distribution in each of the magnetic films, and reduce equivalent resistance. It is therefore possible, in an inductive element, to reduce the total sum of values of equivalent resistance of all the magnetic films, improve superposed DC current characteristic, and ensure an inductance equal or superior to the conventional one.